NAND flash memory devices are non-volatile memory devices. When the distribution of threshold voltages of NAND memory cells included in a NAND flash memory device is measured, a page is read multiple times. The data bits of the page read under different read operations are compared with each other, and the variation of distributions is measured using the comparison result. Such comparison and measurement is performed by a memory controller that controls the operation of the NAND flash memory device. For instance, the memory controller requires two sets of buffers for storing data read in two read operations, respectively, and an XOR logic circuit for comparing data output from the two sets of buffers bit by bit. However, due to time taken to read data from the NAND flash memory device in two read operations, to transmit the data to the memory controller and to perform a bit-wise XOR operation on the data, it takes long time for the memory controller to measure the variation of the data.